Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise a compressor, a condenser, an expansion device, e.g. in the form of an expansion valve, and an evaporator, arranged in a refrigerant path. Refrigerant flowing in the refrigerant path is alternatingly compressed by means of the compressor and expanded by means of the expansion device, and heat exchange takes place in the condenser and the evaporator. In the condenser heat is rejected from the refrigerant flowing through the condenser, and in the evaporator heat is absorbed by the refrigerant flowing through the evaporator. Thus, cooling or heating can be provided to a closed volume arranged in thermal contact with the evaporator or the condenser, respectively.
As described above, refrigerant is expanded by the expansion device before being supplied to the evaporator. Therefore the refrigerant being supplied to the evaporator is in a mixed gaseous and liquid state. In the evaporator, the liquid part of the refrigerant is at least partly evaporated, and the refrigerant absorbs heat due to this phase transition. If all of the liquid refrigerant is evaporated before it reaches the end of the evaporator, the gaseous refrigerant is heated, and the refrigerant leaving the evaporator has a temperature which is higher than the dew point of the refrigerant. The temperature difference between the temperature of the refrigerant leaving the evaporator and the dew point is referred to as the superheat of the refrigerant.
It is desirable to ensure that liquid refrigerant is present in the evaporator along the entire length, i.e. that the superheat of the refrigerant leaving the evaporator is zero or close to zero, because thereby it is ensured that the energy consumed is spent on evaporating refrigerant, thereby providing cooling, rather than on heating the gaseous refrigerant. Thus, a low superheat value ensures that the refrigerating capacity of the vapour compression system is utilised to the greatest possible extent.
On the other hand, liquid refrigerant should not be allowed to leave the evaporator, to such an extent that liquid refrigerant reaches the compressor, because this may lead to damage of the compressor. Therefore vapour compression systems are normally operated in order to obtain a small, but positive, superheat of the refrigerant leaving the evaporator. This is normally done by operating the expansion valve, thereby controlling the supply of refrigerant to the evaporator.
In some situations, such as during start-up of a vapour compression system, the evaporation temperature may be very low. In this case, when controlling the expansion device in order to obtain a positive superheat value, the suction pressure may become very low, because the supply of refrigerant to the evaporator is reduced in order to prevent liquid refrigerant from passing through the evaporator. A low suction pressure is also undesirable, and in order to increase the suction pressure, it is necessary to increase the supply of refrigerant to the evaporator. Thus, there are two conflicting control strategies: One requesting that the supply of refrigerant to the evaporator is decreased in order to prevent liquid refrigerant from passing through the evaporator, and one requesting that the supply of refrigerant to the evaporator is increased in order to increase the suction pressure. In such situations, the control strategy requesting that the supply of refrigerant to the evaporator is decreased will normally be selected, because it is considered more important to prevent potential damage to the compressor than to increase the suction pressure. However, this may result in an inefficient start-up of the vapour compression system.
U.S. Pat. No. 5,077,983 discloses a method and apparatus for improving efficiency of a pulsed expansion valve heat pump. During normal operation the expansion valve is pulsed to control the flow of refrigerant to the evaporator component. The expansion valve is pulsed under the control of a proportional integral algorithm so as to control the compressor discharge temperature to maintain it at a target temperature which maximizes the steady state system efficiency. During start-up of the compressor the discharge temperature control of the expansion valve is interrupted, and a constant duty cycling pulsing of the expansion valve is substituted until the discharge temperature reaches a predetermined value.